1. Field of the Invention
The present invention relates to a process for removing organic material from wet process phosphoric acid.
2. Description of the Prior Art
In the present specification, crude phosphoric acid prepared by treating a phosphate rock with a mineral acid such as sulfuric acid, hydrochloric acid, nitric acid or the like, and phosphoric acid prepared by using said mineral acid are referred to as wet process phosphoric acid. Such wet process phosphoric acid contains a large amount of organic material together with metallic component impurities such as Fe, Al, Ca and the like which are present in the phosphate rock and the mineral acid. Also present are excess mineral acid, a fluorine component and a silica component. As a result, the utility of the wet process phosphoric acid has been limited.
However, it has been proposed to purify wet process phosphoric acid so that it can be used to prepare industrial products, medicines, foods, and the like by the extraction method. This purification process includes extracting the wet process phosphoric acid with an organic solvent which dissolves phosphoric acid and which has low solubility in water. Such solvents include alcohols, e.g., n-butanol, isobutanol, isoamyl alcohol; ketones, e.g., methylisobutyl ketone; ethers, e.g., isopropyl ether; phosphoric acid esters, e.g., tributyl phosphate; amines and the like. Thereafter, phosphoric acid is extracted from the extracted solution of phosphoric acid with water. This extraction method has been used in industrial operation. In this specification, a phosphoric acid prepared by purifying the wet process phosphoric acid by the extraction method, including those prepared with a pretreatment such as treatments for removing mineral acid and/or fluorine components before the extraction, and those prepared by further concentration, is referred to as extracted phosphoric acid.
Phosphoric acid which contains a large amount of organic material has a brown to black color and cannot be used without purification. It is necessary to decolor it. Various decoloring methods, such as adsorption treatments with active carbon, ion-exchange resins and the like; decomposition with an oxidizing agent; and extraction with a solvent and the like, have been proposed. As disclosed in Japanese Unexamined Patent Publication No 18388/1975, these methods are effective for decoloring crude phosphoric acid. However, when the phosphoric acid prepared by these methods is heated to concentrate it, further coloring results. The present inventors have investigated this phenomenon of recoloring caused by heating, and have made the following observations.
A. The organic materials in a wet process phosphoric acid can be classified into those which cause coloring and those which do not.
b. The non-coloring organic materials can be converted into coloring organic materials by heating.
c. Accordingly, the phenomenon of recoloring caused by heating cannot be prevented without all of the organic materials in the phosphoric acid being removed.
d. The coloring organic materials can be removed but the non-coloring organic materials cannot be removed by the conventional decoloring methods.
e. The wet process phosphoric acid usually contains 0.01-1 wt% (as carbon) of organic materials depending upon the source and type of the phosphate rock and the manner of preparation of the wet process phosphoric acid. The non-coloring organic materials represent about 30-50% of the total organic materials.
f. A portion of the coloring organic materials can be removed but the non-coloring organic materials are not substantially removed by the extraction method. The facts that non-coloring organic materials are present, that they are hard to remove and they can be converted to the removable coloring organic materials by heating have been discovered by the present inventors.
The heating conditions have been further investigated and it has been determined that the conversion starts at higher than 130.degree. C and can be completed at higher than 200.degree. C. When the phosphoric acid is heated to such a high temperature, corrosion of the apparatus is severe and polyphosphoric acid is formed. Accordingly, the process of heating the phosphoric acid to convert the non-coloring organic materials to the coloring organic materials and then decoloring it by a conventional decoloring method, can attain the purpose of decoloring but may not be used industrially. Consequently, it is highly desirable to decrease the temperature for heating and the present inventors have found that to be possible under special conditions.